Falls and their consequences are among the major problems in the medical care of older individuals. Our central hypothesis is that age-associated abnormalities of excessive and poorly habituated startle/postural FTRs increase fall risk and related injuries by interfering with effective postural movements through excessive muscle responses and joint stiffening, and by increasing injurious fall impact forces, or both. However, such abnormalities with older age may be modified through residual startle plasticity capacity. During sudden imbalance, unexpected postural disturbances resemble startling events that may precipitate a fall. The excessive startle-like reaction incorporated into the first trial postural response (FTR) normally diminishes with habituation. Because all falls include downward motion of the body with gravity that may trigger a startle, understanding whether the FTR during sudden drop perturbations either enhances protective balance recovery or is problematic for it would be important to determine. Furthermore, whether the FTR is changed with aging would also be essential to know because both balance and startle show age-related abnormalities. Moreover, identifying ways to therapeutically normalize abnormal startle contributions to FTRs with aging by determining the modulatory influences such as behavioral habituation, subject awareness through motor prediction (central set) that normally modifies startle responses, or through a sensory prestimulus before the drop perturbation (prestimulus inhibition) would be important to identify. Whole body postural muscle activation patterns, movement kinematics, and landing impact forces will be assessed by electromyographic (EMG) recordings, motion capture, and force platform recordings. The specific aims are 1) to determine the age-associated changes in neuromuscular and kinematic mechanisms affecting startle/postural FTRs and response habituation after externally-activated (reactive) drop perturbations of the standing support surface by measuring the timing and magnitude parameters indicative of impaired sensorimotor conduction and brainstem processing and integration of somatosensory and vestibular information with older age; and 2) To determine a) the modulatory effects of motor prediction in relation to older age on FTRs by comparing the whole-body startle/postural responses to reactive drop perturbations and self-activated (predictive) perturbations on FTRs and response habituation, and b) the modulatory effects of an acoustic prestimulus on the FTR magnitude and subsequent postural responses evoked by reactive drop perturbations. Expected outcomes are that abnormal neurophysiological mechanisms causing excessive startle contributions to postural FTRs with older age will be linked with impaired balance stability. Moreover, determining the acute modulatory influences on startle will be first steps in the development of novel future rehabilitation interventions to minimize disruptive FTR reactions and prevent falls and related injuries.